I'm sure you will get lots of answers so I won't attempt to cover all of your questions, but do have a few comments.
Due to the high pixel density of DSLRs, their best match is to optics of, say, 1200mm or less focal length. Beyond that focal length and you are over-sampling unless you use focal reducers. Beyond about 2000mm focal length you will lose your ability to effectively use most focal reducers.
For your budget I'd personally recommend a 150 to 200mm aperture newtonian, or a 80-100mm apo refractor if you want to take the less stressful road. Examples are
https://www.bintelshop.com.au/Product.aspx?ID=8471 for the newt and mount, or the same mount with an ED80 or similar refractor on top. For the newt you would need a coma corrector. For the refractor you would want a field flattener. Note that this size of refractor will not give good results with smaller DSOs, such as planetary nebula, and is more of a wide field configuration. However, it is much more forgiving than a newtonian or SCT would be to learn on.
If you want to do guiding using a second OTA, you'd be needing either a side by side or piggyback setup for the guide scope, plus a guide camera, plus the guide scope. Realistically adding guiding of this type will add $750-1000 to the cost of the setup as a result, and also can push the weight limits of the mount.
Regards,
Eric